Stevia is a perennial plant of the Compositae Asteraceae originally grown in Paraguay, South America, and its scientific name is Stevia Rebaudiana Bertoni. Stevia contains sweet components having the sweetness of 300 times or more than that of sugar, and is planted for use as a natural sweetener obtained by the extraction of this sweetening components.
As the sweetening components, Stevioside (C38H60O18), Rebaudioside A (C44H70O23), Rebaudioside C, D and E, Dulcoside A etc. have been known. With the variety generally planted, Stevioside (ST) is the major component among the above-mentioned sweet components with a content amount of Rebaudioside A (RA) being about 3/10 to 4/10 that of Stevioside and that of Rebaudioside C being slightly less, but depending on varieties, there is Stevia with Rebaudioside C being the major component. That is, there are various varieties.
Among the tastes such as bitterness and astringency, sweetness is very delicate. Since Stevioside has a degree of sweetness of 300 times that of sugar, it has been used as a natural sweetener in the food industry. Its sweetness is relatively similar to that of sugar, however, there is a defect in that an unpleasant taste such as bitterness remains in the mouth. Therefore, it is not desirable for a sweetener to contain a large amount of Stevioside. On the other hand, Rebaudioside A has sweetness of a good quality and a degree of sweetness of 1.3 to 1.5 time that of Stevioside.
It is thus necessary to reduce the production cost of Rebaudioside A, to maintain the stable yield of dried leaves, to develop a variety of Stevia which contains a high content amount of Rebaudioside A having excellent sweetening quality as a sweetening raw material, and at the same time, to maintain its continuous supply and to produce an excellent sweetener based on these.
The inventors of the present invention carried out plant breeding through the repetition of selective cross fertilizations of conventional varieties, thereby obtaining Stevia varieties with a high content ratio of Rebaudioside (RA) to Stevioside (ST), and sweeteners excellent in content ratio of Rebaudioside A to Stevioside (ST) have been produced by extracting sweetening component from these plants (see Patent Literatures 1-1 through 1-3 to be described later), however, development of varieties stably with a higher and better content ratio of Rebaudioside A has been desired.
On the other hand, in plant breeding of Stevia plants, an issue is a method of identification of Stevia plants. As an identification method of improved Stevia plants, we may think of identifications by plant heights, shapes of leaves, etc., however, as Stevia is self non-compatible and tends to become hybrids, classification only by plant heights, shapes of leaves, etc. can not achieve this.
In addition, there exist comparative identification based on the disease resistance to pathogens specific to Stevia, however, although dead leaves and black leaf spots which occur specifically to Stevia are caused by Septoria fungus and Alternaria, these fungi live in soil. Therefore, since these symptoms occur not only in Japan but also throughout the world, these characteristics alone are insufficient for the identification of a variety.
With an improved variety with a high sweetening content and a high content ratio of Rebaudioside A in comparison with Stevioside, one may think of identification method by this content ratio, however as a variation in sweetening content ratio may inevitably vary depending on a weather condition during the growth period, a harvesting time, etc., this method lacks practicality.
Recently, a method by which to make identification through DNA identification based on the RAPD method which uses a primer mix (see Patent Reference 2 below) has been developed, however, it is not clear whether or not it is possible to apply this to the identification of a plant in accordance with the present invention.    Patent Reference 1: Laid Open Patent Publication Sho. 59/1984-045848 Gazette    Patent Reference 2: Laid Open Patent Publication Sho. 60/1985-160823 Gazette    Patent Reference 3: Laid Open Patent Publication Sho. 61/1986-202667 Gazette    Patent Reference 4: Laid Open Patent Publication No. 2003-9878 Gazette